North Suburban HAMMOND ORGAN Society

We begin with a general description of a typical Leslie. The musical audio signal enters the input stage of a power amplifier where its level is increased to drive the speakers. In a typical Leslie, there are two speakers, however there are many variations on the "typical" Leslie speaker which are designed for specific electronic organs and their particular requirements. We shall touch upon some specific Leslie variants later in this article. For the time being, we'll consider a basic, generic Leslie such as that which we might use with a typical Hammond console.

After amplification, the signal goes to a crossover network with a crossover at 800 Hz. Bass tones go to a fifteen inch woofer, and the treble tones >800 Hz go to a compression type driver. The treble driver's sound output enters the narrow end of an exponentially tapered horn; the woofer's sound output enters a "scooplike" baffle. Both the treble horn and the scoop baffle can rotate, driven by small induction motors through belts and pulleys so that they spin at nominally 370 RPM.

From reading our article on vibrato, we know that 370 RPM is 6.17 revolutions per second. Each revolution of the Leslie horn or baffle represents one complete cycle of vibrato, and likewise from that article, we know that the most generally pleasing rate for vibrato is between six and seven vibrato cycles per second.

In many Leslie speakers, there's a second, small motor associated with each regular motor and which can drive the regular motor shaft through a rubber tire on a second pulley at a slower speed, so that the treble horn and bass rotor turn a little under one revolution per second. This mode of operation is intended to produce an effect similar to the slow, random wavering that occurs in real pipe organs as a result of their lack of absolute tuning perfection. It is physically impossible to achieve or maintain absolute tuning perfection in pipe organs, or in any other real polyphonic musical instrument, and this subtle, random out-of-tuneness makes the resulting sound of the instrument more interesting. It's important to note that this slight tuning error is very subtle. Individual tones are still so close in pitch that the instrument sounds in tune, and many people are not even aware of the effect of the slight tuning errors until it is specifically pointed out to them.

In an instrument such as the traditional Hammond where all generated pitches are sine wave tones without any upper harmonics, and where all of the harmonics of the instrument's musical output tones are derived from the equally tempered scale, and where all octaves are locked through gearing into an exact 2:1 ratio*, the subtle, random effect of slight tuning imperfection is not present; and when compared to the sound of a real instrument with such tuning errors, the usual adjectives that people use to describe the effect are, among others, lifeless, boring, sterile, uninteresting.

The slow rotation of the Leslie rotors does indeed add a similar effect, however the effect is repetitive due to the regular rotation of the rotors. However, it certainly does contribute a degree of interest to certain registrations of an electronic organ and is a significant improvement if you want to eliminate the lifeless effect of perfectly synchronized octaves and tones with sine wave harmonics derived from the tempered scale.

The majority opinion however is that the Leslie effect is significantly better when applied to tones which are sine waves or combinations of sine waves from the tempered scale and used as harmonics. The effect is not that good on electronic instrument sounds which are generated as true complex waveforms with their own natural harmonics, and thus in many electronic organs, you will find the Leslie effect applied only to sine wave tones and their derivatives, and complex waveform tones are not subject to the rotary treament. (Our group's X66 is a classic example of this, where the Leslie speaker we have on that instrument handles only the drawbar "A" channel [sine wave, synthetic harmonic tones] and the rest of the instrument's output is heard only through the standard X66 speaker cabinet.)

*To be technically accurate, the highest seven tones on a traditional Hammond are not exactly a 2:1 octave higher than their corresponding lower tones. In a traditional Hammond, these tones are produced by tonewheels with THREE times more teeth or lobes than the other tonewheel on the same shaft of each tonewheel pair, and represent the TRUE third harmonic of the pitches generated by the lower-pitched tonewheels of those wheel pairs. The actual error in pitch by doing this is very slight; the fact that all of the tonewheels produce sine waves anyhow means that this error is not either noticeable or heard. The only practical way to discover it is to look at these waveforms on an oscilloscope as a real-time display, and then the slight error shows up as a slight movement in the display on the screen. In a similar manner, the very top C of the X66 model is generated by a special circuit from the third harmonic of the next to the top F. Likewise, its slight tuning error is not detectable except by scope measurements.

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